4 replies to this topic

[polipo]

Dear guys,

I'm wondering whether henry's law constant for CO₂ in NaOH solution is equal to CO₂ in water or not. If not, could anyone tell me where I can find this constant for CO₂ in 0.3 M NaOH solution at 30 ^oC? or any formula to calculate?

Thank you.

Edited by polipo, 04 July 2010 - 12:12 AM.

[kkala]

I'm wondering whether henry's law constant for CO₂ in NaOH solution is equal to CO₂ in water or not. If not, could anyone tell me where I can find this constant for CO₂ in 0.3 M NaOH solution at 30 ^oC? or any formula to calculate?

I believe Henry's constant remains same as that of pure water. Of course ionic equilibrium in liquid phase is affected by dissolved NaOH.
For CO2 in water at 30 oC, H=1860 atm/mole fraction (Foust, Wenzel,"Principles of Unit Operations", Wiley, 1960, Pco2=H*xco2).
For dissolved CO2, ionic equilibrium can be presented (in 3 equations) as
CO2+H2O = HCO3^-+H⁺ (pKa=6.37 at 25 oC)
HCO3^-+H2O = CO3^2-+H⁺ (pKa=10.25 at 25 oC)
H2O = H⁺+ OH^- (water dissociation, pK=14 at 25 oC)
For first two equations see the reference "Carbondioxide in waterequilibrium" in http://www.thuisexperimenteren.nl, or find it by googling "Henry constant CO2"; the reference gives general help on this issue.
Consequently four equations (including that of Henry's constant) have to be solved, considering what is variable and what remains constant. E.g. if NaOH concentration remains constant at 0.3 M, the reference indicates practically only CO3^2- in liquid phase (thus practically zero CO2 in Gas phase), when equilibrium is reached.

Edited by kkala, 04 July 2010 - 05:22 AM.

[polipo]

So, can I used that value of Henry's law constant?
Anyway, I've used that value to determine NTU for the gas absorber, but it can't be determined. When I plotted the graph of the equilibrium curve and operating curve, it intersected each other.

[kkala]

So, can I use that value of Henry's law constant? Anyway, I've used that value to determine NTU for the gas absorber, but it can't be determined. When I plotted the graph of the equilibrium curve and operating curve, it intersected each other.

Probably another check is worth while. I cannot be specific, not having theoretical experience on the field of gas absorbtion.
I believe previous post is applicable. According to it, soda solution as an absorbent requires more complex calculations compared to the case of water absorbent. CO2 gas is absorbed into liquid (Henry's law) and this absorbed CO2 gets in equilibrium with soda, according to previous (today) post. Not only Henry constant but also dissociation constants have to be used.
When free OH^-- is present in liquid (e.g. for constant NaOH concentration = 0.3 M), CO2 in liquid is practically zero and so is partial CO2 gas pressure at equilibrium. Thus efficiency of CO2 removal from gas depends only on kinetics (contact surface and time) in this case.

Edited by kkala, 04 July 2010 - 11:19 AM.

[siretb]

You have to be careful.
The true Henri's constant P=H*xCO2 is about the same, save for the ionic strength, but as soon as a CO2 molecule is deissolved into a high pH solution, the equilibrium transforms it into CO3-- and HCO3-.
Therefore you cannot calculate the operating and equilibrium lines as if it were pure water.
Perry's handbook informs well, in chapter "absorption with chemical reaction", 14-17in 7th edition of what happens.
The chemical reaction pulls the transfer and the Kl.a is enhanced, when compared to H2O. This is the so called Hatta factor.

In practice, as long as the pH remains high (say >>9) you may consider that there is no CO2 partial pressure. meaning H=0. (if expressed as P=H*(total CO2)=H*(xCO2+ xCO3-- + xHCO3-), as kkala said well.
This does not mean that is absorption is easy, because NaOH-CO2 is a slow system,and the HTU's tend to be high. Unless you exhaust the buffering capacity of the caustic, the system will not be equilibrium limited, but mass-fransfer limited.
The following value is given in Perry's handbook, for 1.5" Intallox kl.a=38 kmols/hr/m3.

In order to do your own calculations you need the actual operating conditions. For a practical design, do not forget that scaling may occur.